Most of waste plastics hitherto have been dumped by reclaiming lands with the same or incinerating the same, and have never been used as a useful resource. This waste disposal by way of reclaiming the lands has difficulties in the ensuring of sites to be reclaimed and in stable hardening of such sites. On the other hand, the disposal by way of incinerating the waste plastics has disadvantages such as the damage of incinerators, generation of harmful gases and offensive odors, and discharge of CO2. To solve these problems, the Containers and Packaging Recycling Law was instituted in 1995 in Japan, so as to obligate the recovering and recycling of plastics. This trend of recovering and recycling products containing plastics is prevailing in association with the enforcement of a variety of recycling laws.
Under these situations, recently, trials to recycle waste plastics for use as a resource have been attempted. As one of such trials, there is proposed a method of recovering useful oily substances from waste plastics by decomposing the waste plastics through a reaction using supercritical water as a reaction medium. There is also proposed a method of recycling fiber-reinforced plastics used in various structural materials, in which the plastic components in such materials are decomposed by using supercritical water or subcritical water, so as to recover fibers such as glass fibers and carbon fibers for recycling them (cf. Patent Literatures 1 and 2).
By these methods, plastics are decomposed into oily components having lower molecular weights so as to recycle these components as liquid fuels. There is further proposed a method of decomposing plastics, which makes use of a hydrolysis reaction by high temperature water vapor. According to this method, it is possible to decompose the organic polymer components of thermoplastic and thermosetting plastics to some extents.
However, the above methods have a disadvantage in that, since plastics are decomposed in random, the decomposition products are oily materials comprising various components, and thus in that it is difficult to obtain decomposition products with constant qualities. Consequently, a post-treatment for reforming the oily materials by using a catalyst, typically, zeolite, is needed, which results in higher cost. Further, it is difficult to produce petroleum products such as lamp oil and light oil from such reformed oils, and therefore, such reformed oils have not yet been put into practical use. In contemplation of a whole of the global environmental problems such as the exhaust of the petroleum resources and the global warming due to carbon dioxide, drastic countermeasures to decompose and recycle plastics are today unavoidably needed.
Patent Literature 1: JP-A-8-85736 (1996)
Patent Literature 2: JP-A-2000-53801 (2000)